Coast Guard Petty Officer George Degener says the 30-foot aluminum boat capsized just south of Alki Point around 2 p.m. Friday while the commercial vessel was conducting a dive survey.
Three people on board were immediately pulled from the waters and were rushed by boat to Fire Station No. 5 on Seattle's waterfront to be taken to Harbroview Medical Center for treatment of possible hypothermia.
Divers searched the nearby waters for a fourth person who had been on board but was missing. That person was found about an hour later under the boat, officials said.
A Coast Guard helicopter from Port Angeles arrived on scene and hoisted that person on board for a direct airlift to Harborview Medical Center where the victim was listed in critical condition.
Winds were gusting out of the south at 29 mph at the nearby Alki Point Lighthouse at 2 p.m. but it's not known yet what caused the boat to overturn.